To make the butterscotch: In a medium-size, heavy-bottomed saucepot, cook the butter and brown sugar together over medium heat.
Stir with a wooden spoon or carefully whisk (be sure to use the right kind of whisk, an egg whisk, or the sugar will get caught in the wires). The sugar and butter will separate at first, but as the sugar melts it will become easier to blend them together.
Cook and stir until the sugar melts and the butter/sugar mixture just starts to brown; it will start to smoke slightly, as well. This may take 15 to 20 minutes.
Slowly add the cream, a cup at a time.
It will sputter and seize up as you add the cold cream. Cook the butterscotch until the seized sugar dissolves again. Add the rest of the cream, vanilla bean and salt.
To make the custard: In a bowl, whisk together the egg yolks. Slowly add some of the hot butterscotch cream to the yolks as you whisk them. Just add enough to warm the yolks slightly.
Once the yolks are warm to the touch, pour the yolks back into the pot of hot butterscotch cream and gently whisk.
Strain the mixture.
To bake the custard: Preheat the oven to 325 degrees F.
Divide the custard into the eight 4-ounce ramekins. Place them in a water bath and loosely tent with foil. (It is important that the water in the pan is hot and comes halfway up the side of the ramekins, so that it will reach oven temperature quickly. Using cool water will prevent them from baking evenly.)
Bake for 25 to 40 minutes. This will depend on the temperature of the mixture when you put it in the oven. It's best to check them after 25 minutes and gauge how much longer they need. Uncover them, jiggle the pan gently and check to see if they are set. They will move like set gelatin when they are done. Be careful not to over bake them or they will be grainy.
Remove the pan from the oven and allow the custards to cool in the water bath. Once they have reached room temperature, you can remove them from the bath, cover them and refrigerate for at least an hour, but they can be made a few days in advance.
To serve: Spread a thin layer of unsweetened creme fraiche over the top of the custard.
I like the contrast of the unsweetened creme fraiche and the sweet pudding, but you can add a touch of sugar if it is too stark for your taste, or serve it with nothing at all.